Fv1 block in MLV infection. Ecotropic MLVs (e.g. Friend MLV) fall into two categories with respect to their host range: N-tropic strains infect NIH/Swiss mice (brown) much more efficiently than BALB/c mice (black), whereas B-tropic strains display the opposite preference. Based on their susceptibility to N- or B-tropic virus, mice were classified into Fv1n/n (NIH/Swiss) and Fv1b/b (Balb/c) strains. These viruses differ at a single residue at position 110 in CA (presented below the NIH/Swiss mouse). These changes are matched by residues at position 358 in Fv1n and Fv1b proteins (presented below the Balb/c mouse).